The traveling wave tube is a known type of electronic vacuum tube device which is widely used as a component of microwave electronic systems as an amplifier of microwave frequency electromagnetic waves. In the traveling wave tube, the microwave frequency electromagnetic wave is made to propagate along a slow wave structure of the device. The slow wave structure causes the microwave signal to traverse a greater length during the time that it is interacting with an electron beam in the tube than does the electron beam, so that the apparent or phase velocity of the electromagnetic wave appears to the electron beam to be substantially equal to the lower velocity of the electrons in the electron beam. Upon interaction between the electrons and the electromagnetic wave, energy is transferred from the electron beam to the electromagnetic wave, and the electromagnetic wave is thereby amplified.
The present invention is concerned with a type of traveling wave tube known as a coupled cavity traveling wave tube, in which the slow wave structure is formed from a plurality of cavities which are aligned along a common axis. The device also includes an equal plurality of apertures in the walls between adjacent ones of the cavities, with these apertures being aligned along the axis of the device. The electron beam is then projected along the axis of the device through these apertures. Typically, microwave coupling between the cavities is provided by coupling slots in the common walls between the adjacent cavities, with the coupling slots being alternately positioned above and below the apertures through which the electron beam passes. Typical coupled cavity traveling wave tubes are shown, for example, in U.S. Pat. No. 4,103,207 and in the various publications mentioned in that patent.
One of the desirable operating characteristics of a traveling wave tube is its extremely broad band width. Typical traveling wave tubes are capable of amplifying input microwave signals over a band width of an octave or more. However, this very strength of a traveling wave tube also sometimes becomes one of its weaknesses. Because the traveling wave tube does have a broad band width, spurious or undesirable signals are also sometimes developed within the traveling wave tube. The traveling wave tube cannot distinguish between a signal of a frequency within its operating band width which an operator wishes to have amplified and a signal of the same frequency which is present in the device which the operator does not wish to have amplified. The traveling wave tube thus proceeds to amplify both the desired input signal and the spurious signals generated within the traveling wave tube itself, but which are within the frequency range in which the traveling wave tube operates. The traveling wave tube thus has a tendency to go into oscillation at undesired frequencies within its operating band width.
One of the known ways of suppressing or preventing undesired oscillation in a traveling wave tube is to provide distributed loss elements along the slow wave structure of the traveling wave tube to stabilize against these oscillations. These distributed loss elements also provide gain equalization across the spectrum of the traveling wave tube to provide a desired gain vs. frequency characteristic of the traveling wave tube to compensate for any unequal gain characteristic which the traveling wave tube might otherwise have across its band width.
One technique which has been used to provide a distributed loss in a coupled cavity traveling wave tube is to position one or more cylinders of dielectric material along the length of the traveling wave tube, with a portion of the cylinders thus being in each of the coupled cavities. Lossy material is included in the cylinder to serve as the distributed loss along the length of the slow wave structure. One problem with this approach is that it results in essentially the same loss being inserted in each cavity, and it makes it difficient to vary the loss in the individual cavities if this is desired. In addition, as the lossy material absorbs microwave energy, it gets hotter, and in many such applications the cylinders are made hollow to allow a cooling fluid flowing within the dielectric cylinder. Thus, the ends of the dielectric cylinder must be taken outside of the vacuum envelope of the traveling wave tube in order to provide the cooling fluid. This requirement to bring cooling fluids within the vacuum envelope has limited the use of this technique to provide the desired distributed loss along the slow wave structure.